This invention, as indicated above, involves an improved transducer for simultaneous measuring a low dynamic pressure in the presence of a high static pressure. During the testing of jet engines and in many other environments, it is often desirable to measure both the static pressure and the dynamic pressure. The static pressure, in most instances, is usually very high and the dynamic pressure is much lower. The dynamic pressure is also associated with a distinct frequency which occurs at a relatively high rate, for example 5000 cycles or greater. In this manner, the dynamic pressure is usually 20 times less than the static pressure. Hence, to measure static pressure, one requires a transducer with a relatively thick diaphragm so that it can stand the high static pressure. On the other hand, such thick diaphragms have a very poor response to low pressure. Therefore, to measure static pressure and dynamic pressure is extremely difficult unless one uses a thick diaphragm in conjunction with a thin diaphragm. However, if one uses a thin diaphragm, then this diaphragm will rupture upon application of the high static pressure which also contains the dynamic pressure. One can think of the dynamic pressure as a relatively high frequency fluctuation on top of a relatively high constant static pressure. Thus, as one can ascertain, using a thick diaphragm to measure dynamic and static pressure is not a viable solution.
U.S. Pat. No. 6,642,594 entitled, “Single Chip Multiple Range Pressure Transducer Device” which issued on Nov. 4, 2003 to A. D. Kurtz, the inventor herein and is assigned to Kulite Semiconductor Products, Inc., the assignee herein, discloses the problems with transducers responsive to large pressures utilized to measure low pressures. Thus, when you have a pressure transducer adapted to measure relatively large pressures, these transducers suffer from a relatively poor resolution or sensitivity when measuring relatively low pressures. This is because, as a span of the sensor increases, the resolution or sensitivity of that sensor at the low end of the span decreases. An example of various piezoresistive sensors are indicated in that patent. In the patent, there is shown different transducers which have thinned regions having the same thickness, but different planar dimensions. In this manner, the thinned regions will deflect a different amount upon application of a common pressure thereto, whereby when excited each of the circuits provides an output indicative of the common pressure of a different operating range.
As indicated above, during the testing of jet engines there is a very high static pressure which, for example, may be 100 psi. Present with the static pressure is a low dynamic pressure, which may exhibit frequencies in the range of 5000 Hz and above. As indicated, using a high pressure sensor to measure the static pressure will give an extremely poor response to the dynamic pressure because of the small magnitude of dynamic pressure which can be, for example, about 5 psi. Therefore, it is desirable to use a relatively rugged pressure transducer having a thick diaphragm to measure static pressure and to utilize another transducer on the same chip having a thinned diaphragm to measure dynamic pressure. Because the thinned transducer is exposed to static pressure both on the top and bottom sides, the static pressure cancels out and does not, in any manner, afford to cause the thinned diaphragm to deflect. According to the present invention, both static and a dynamic pressure is applied to the rear side of the diaphragm by a reference tube of an exceedingly long length. This reference tube, as will be explained, is a helical structure and has a low resonant frequency. In this manner, when a small dynamic pressure is applied because of the low internal frequency of the tube, the sensor will respond to the static pressure only. As will be explained, the thinned diaphragm should be stopped for pressures in excess of 25 psi, or some higher number than the desired dynamic pressure. As will be explained, the long reference tube can be made by taking a tubular structure and wrapping it such that it looks like a coil or spring. One end would be inserted into the transducer and the other end would be exposed to pressure. In this manner, one can now implement an improved transducer for simultaneously measuring a low dynamic pressure in the presence of a high static pressure.